The money would come from the appropriate year's budget, though, not the current one (which covers spending between 1 Feb 2011 and end Jan 2012).
Oh, of course, however any spare money from the underspend will carry over to the next budgets - was just suggesting this could be a future outlet for it if we are still racking brains for what to do with it :) There seems no harm in earmarking a potential Wikimania fund now and putting underspend in it by agreement.
BTW, Seddon started a draft bid page for 2013 in Manchester, which is at:
http://uk.wikimedia.org/wiki/Manchester_Wikimania_Bid
that might form a useful basis to start developing an official bid, either for 2013 or later.
Looks interesting; Manchester is a nice city & fairly accessible. It would be good to put together ideas for venues and assess the relative benefits of each of them (i.e. costs, accessibility, local wikipedians).
Tom